1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same, and more particularly, to an image sensor for converting optical signals into electrical signals, and a method of manufacturing the same.
2. Description of the Related Art
Image sensors are semiconductor devices that generally convert optical signals into electrical signals. Examples of image sensors include complementary Metal Oxide Semiconductor (CMOS) image sensors (CIS) and Charge Coupled Device (CCD) image sensors. CIS and CCD image sensors share some similarities with one another with respect to both structure and function. For instance, light incident to two-dimensionally arranged photodiodes in both the CIS and CCD image sensors is converted to a signal charge (electrons), and is then consecutively read out as a signal voltage according to a temporal axis. However, the CIS image sensor differs from the CCD sensor with respect to converting a signal charge into a signal voltage, as well as with transferring the signal to an output port. For example, a CIS image sensor converts a signal charge to a signal voltage in a plurality of unit pixels, and outputs a signal from a signal line by a switching operation. A CCD, however, transfers a signal charge in order of a vertical register and a horizontal register, and converts it into the signal voltage just before an output port.
Additionally, the CCD is partitioned into a pixel region including a photodiode that converts incident light into a charge enabling image processing, and a peripheral circuit region that controls pixels. In the pixel region, an active region is defined by a device isolating layer, and a photodiode and a plurality of gates are formed on the active region. In the peripheral circuit region, elements that control the pixels are formed on an active region defined by the device isolating layer.
FIG. 1 is a cross-sectional view illustrating a pixel of a conventional image sensor.
Referring to FIG. 1, a device isolating layer 15 having a Shallow Trench Insulator (STI) structure is formed on an n-type semiconductor substrate 10. A photodiode 20, a transfer gate 30, and a floating diffusion region 40 are disposed in an active region defined by the device isolating layer 15. The transfer gate 30 includes a conductive layer 35 for a gate and a gate insulating layer 33. The transfer gate 30 transfers optical charges generated from the photodiode 20 to the floating diffusion region 40. An interlayer insulating layer 50 is formed on a pixel region including the photodiode 20 and the transfer gate 30. The interlayer insulating layer 50 is typically composed of an oxide.
In the conventional image sensor illustrated in FIG. 1, a phenomenon known as dark current may occur due to crystal defects or dangling bonds being generated on the surface of the semiconductor substrate 10 or a bordering surface between the device isolating layer 15 and the semiconductor substrate 10. A dark current is a current which is produced even when no light is supplied to a photodiode of an image sensor. The above-mentioned dark current may cause the picture quality of the image sensor to deteriorate.
Moreover, with the above-mentioned conventional image sensor, the dark current may also flow to another photodiode instead of the photodiode 20 in the active region defined by the device isolating layer 15, so that the corresponding photodiode 20 may not form an accurate signal which is based upon the actual amount of light received by the micro-lens of the photodiode 20 of the image sensor. Furthermore, crosstalk may occur between neighboring photodiodes of an image sensor, which in turn may impede the formation of a desired image.
Thus, there is a need for an image sensor that prevents dark current and crosstalk, as well as for a method of forming the same.